The second in the line of succession could be entitled to over £10 million (Rs 86 crore) when he reaches the milestone on June 21.
“It will be quite a handy sum if he wants to buy his and Kate’s first family house. What else would he need the money for?”a source was quoted as saying.
The Royal can also vary his mother’s will to allow brother Harry a greater share of the capital — in recognition that William will one day inherit the income from the Duchy of Cornwall when his father becomes king.
Her estate comprises stocks and shares, jewellery, cash, dresses and other personal belongings at her Kensington Palace apartment.
Diana stipulated that it would be held in trust to be shared equally between her two sons.
In her original will, drawn up in 1993, the princess had stipulated that both princes would be entitled to their entire share of the capital on reaching 25.
But details of the will were changed by a variation order granted by the High Court on December 19, 1997 — three months after her premature death in a Paris car crash.
In a highly unusual move, the executors made both her original will and the new one public.
The variations to the princess’s will also stated that her wedding dress, wardrobe and intellectual property rights — including copyright, trademarks and royalties from the use of her pictures and name — can be used for charity and also for the future benefit of her sons.
“It is a private matter,” a spokesman for Clarence House said.